How do I set all photos in a project to the same date (rather than simply time-shifting)?

I have a bunch of old scanned photos in Aperture (current version - 3.5.1), and am trying to tidy up the metadata, especially the dates.

This is turning out to be quite a task, as the date gaps between scans are all over the place. For example, in one project I might have photos scanned between 2004 and 2007 that all took place on one day in 1999.

This means the native "adjust date and time" function in the metadata menu doesn't work, as it is a little too clever in its timeshifting. If I highliht the whole project and change it all to 1 June 1999, the 2004 shots get changed to that date but the 2007 scans are dated 2002, with Aperture "cleverly" preserving the inaccurate three year gap in the original scans' metadata.

This is making the process of fixing my data far more arduous than it needs to be.

I feel like my situation can't be that unusual, and their must be a way to override this behaviour. I'm not fussy about preserving logical time gaps between shots but I'd like them to have the right year and month (so a "dumb" solution that gave them all idntical date and times would be fine).

Can anayone help? Thanks in advance!